FIG. 13 is a sectional view showing the major parts of a related-art image-scanning apparatus 131. The image-scanning apparatus 131 includes a casing 132 and an image-scanning unit 133 disposed in the casing 132. Contact glass 134 on which a document is placed is provided on the upper surface of the casing 132. The image-scanning unit 133 includes a contact image sensor (CIS) 136 and a carriage 135, and is constructed so that the contact image sensor 136 is held on the carriage 135. A long-sized guide shaft 138 is constructed on the casing 132. The guide shaft 138 is composed of a round bar-shaped metallic axis. The carriage 135 is supported movably and slidably in the direction perpendicular to the paper of the same drawing. A linkage portion 139 is provided at the lower part of the carriage 135, and a timing belt of a drive mechanism (not illustrated) is connected to the linkage portion 139. Therefore, the image-scanning unit 133 can scan an image of a document by scanning the document placed on the contact glass 134 while moving in the above-described direction. A roller unit 137 is provided at both end sides of the image-scanning unit 133. The roller unit 137 is contact with the rear side of the contact glass 134 and supports smooth movements of the image-scanning unit 133.
FIG. 14 is a detailed enlarged view of a supporting portion (portion enclosed by a broken line in FIG. 13) of the carriage 135 by the guide shaft 138. A groove 140 having a larger width than the diameter of the guide shaft 138 is provided at the lower part of the carriage 135. The bottom side of the groove 140 has a horizontal plane 143 and a pair of inclination planes 142a and 142b inclined so as to have a fan shape from both ends of the corresponding horizontal plane 143, and perpendicular planes 141a and 141b continued to the inclination planes 142a and 142b, respectively, are formed to the lower side of the carriage 135. The carriage 135 is supported by the groove 140 being fitted onto the guide shaft 138. The carriage 135 is supported at two points of tangential lines 144a and 144b (that is, the tangential lines extending in the perpendicular direction of paper in FIG. 14) of the circumferential surface of the guide shaft 138 and the inclination planes 142a and 142b. Therefore, there is no case where any backlash is generated in the radial direction of the guide shaft 138.
However, JP-A-5-147300 discloses that a supporting structure supports the carrier frame (11) by fitting a round bar-shaped guide shaft (14) into the carrier frame (11) having an inverted U-shaped sleeve (13). (See FIG. 1 of JP-A-5-147300.) However, with the corresponding supporting structure, it is necessary to increase the width of the sleeve (13) larger than the diameter of the guide shaft (14) in order to movably and slidably support the carrier frame (11). For this reason, play is generated between the surface of the sleeve (13) and the circumferential surface of the guide shaft (14), and backlash is brought about in the radial direction of the guide shaft (14). This backlash is not preferable in view of producing distortion in a read image. Since the sleeve (13) slides while keeping surface contact with the guide shaft (14), the sliding resistance is large, resulting in a problem by which smooth sliding movement is hindered. Therefore, in recent years, the supporting structure shown in FIG. 13 and FIG. 14, which is described above, has been mainly employed. Also, in the above description, reference numerals used in FIG. 1 of JP-A-5-147300 are shown in brackets for convenience.
When the supporting structure of the carriage 135 shown in FIG. 13 and FIG. 14 is employed, it is necessary to set the clearance between two tangential lines 144a and 144b more than a predetermined width in order to achieve stable sliding movement of the carriage 135. The inclination planes 142a and 142b are necessarily spaced, and also the width of the groove 140 are widened. The diameter of the guide shaft 138 is increased. However, in case that the diameter of the guide shaft 138 is increased to become large, the height of the supporting structure is necessarily elongated in the perpendicular direction, and the scale of the supporting structure is made large-sized.